Implement customizable transition animation duration setting

[sarthak-19]

Part of a polishing issue : #2317

Summary

This PR implements the transition animation duration setting that was previously added to the UI but wasn't functional. The duration setting now properly affects view transitions on both the frontend and WordPress admin area.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.65%. Comparing base (ce1b405) to head (4970c8d).
⚠️ Report is 2 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/view-transitions/includes/settings.php	0.00%	34 Missing ⚠️
plugins/view-transitions/includes/admin.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2321      +/-   ##
==========================================
- Coverage   68.85%   68.65%   -0.21%     
==========================================
  Files          90       90              
  Lines        7612     7643      +31     
==========================================
+ Hits         5241     5247       +6     
- Misses       2371     2396      +25     

Flag	Coverage Δ
multisite	68.65% <0.00%> (-0.21%)	⬇️
single	35.40% <0.00%> (-0.07%)	⬇️

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[westonruter]

Comment from @sarthak-19 in Slack:

I raised a draft PR because currently I was unsure of what more are we expecting to be delivered in this task for polishing, since the issue mentioned of Add support for customizing transition animation duration
But this was already added in the UI as well.
What I did for potential polishing items are :

1. Added validation bounds - Duration is now clamped between 100ms and 5000ms with proper input validation
2. Applied duration to admin - Admin view transitions now also respect the duration setting
3. Added real-time preview - The seconds equivalent updates dynamically as you type in the > milliseconds field.
4. Exposed in REST API - Added default_transition_animation_duration to the REST API schema
5. Added unit tests - Tests for duration sanitization, clamping, and the injection function